Power devices are extensively used as switching elements for different kinds of power sources and for electric devices of motor vehicles or as switching elements for electric devices of industrial machines. As compared with a typical semiconductor device, the power devices are configured to enjoy a high voltage resistance, a high current, a high speed and a high frequency. As the power devices, there are available an IGBT, a diode, a power transistor, a power MOS-FET and a thyristor. These power devices are subjected to electric characteristic test regarding the static characteristics and the dynamic characteristics (switching characteristics) thereof and are then mounted as electronic parts depending on the applications thereof.
A diode is parallel-connected to, e.g., a power MOS-FET, and is used as a switching element of a motor and so forth. The diode has a depletion layer existing in the juncture between an anode and a cathode. It is sometimes the case that the switching function inherent in the diode is impaired by the depletion layer. In particular, if a reverse voltage is applied to the diode while a current IF flows, the current is sharply decreased to zero as indicated by, e.g., a solid line (an ideal value) in FIG. 13. In reality, however, a small amount of carriers remain in the depletion layer. Therefore, if a voltage is applied in this state, a large countercurrent IR flows as indicated by a solid line in FIG. 13. The countercurrent reaches the maximum countercurrent value Irp and then comes back to zero. The time required for the countercurrent to be recovered from the maximum countercurrent value Irp to a current value equal to one tenth of the maximum countercurrent value Irp is defined as a reverse recovery time trr. From the viewpoint of the switching characteristics of the diode, it is desirable that the reverse recovery time be short. If the reverse recovery time is long, the diode is sometimes destroyed depending in the use conditions. As the current change (di/dt) of the countercurrent becomes steep, the current grows larger and the diode tends to be destroyed with ease. This destruction is called di/dt destruction. For that reason, the current change (di/dt) in the diode generated when switching the current is measured by mounting the diode to a dedicated measuring instrument, thereby evaluating the reliability of the diode as a switching element.
The present applicants have conducted a variety of studies on a method in which the current change (di/dt) in a diode included in individual power devices of a semiconductor wafer is measured under a wafer state through the use of, e.g., a probe apparatus shown in FIG. 14. The probe apparatus 110 shown in FIG. 14 includes a loader chamber (not shown) within which the semiconductor wafer is transferred and a prober chamber 111 within which the electrical characteristics of the semiconductor wafer transferred from the loader chamber are tested. The probe apparatus 110 is configured to test the electrical characteristics of a power device under a wafer state.
As shown in FIG. 14, a movable mounting table 112 for holding a semiconductor wafer W and a probe card 113 arranged above the mounting table 112 are provided in the prober chamber 111. A conductive film electrode made of an electrically conductive material such as gold or the like is formed on the surface of the mounting table 112. The conductive film electrode is electrically connected to a tester 115 via a cable 114. The probe card 113 includes a plurality of probe pairs 113A Kelvin-connected to individual electrode pads of the semiconductor wafer W. The probe pairs 113A are electrically connected to the tester 115 via force lines 116F and sense lines 116S. The Kelvin-connection of the probe pairs 113A makes it possible to eliminate measurement errors which may be caused by the contact resistance between the probe pairs 113A and the electrode pads and the internal resistance of the respective lines 116F and 116S.
As set forth above, a plurality of power devices are formed in the semiconductor wafer W. Each of the power devices includes, e.g., a MOS-FET (or an IGBT) and a diode, both of which are parallel-connected to each other. The power devices are used as switching elements. A gate electrode and a source electrode of the MOS-FET are formed on the upper surface of the semiconductor wafer W. A drain electrode is formed on the lower surface of the semiconductor wafer W. The conductive film electrode of the mounting table 112 making contact with the drain electrode serves as a drain electrode. The cable 114 connected to the drain electrode includes a force line 114F and a sense line 114S. The cable 114 is Kelvin-connected to the conductive film electrode of the mounting table 112, in which state the cable 114 is connected to the tester 115. In case of the IGBT, the respective electrodes thereof include a gate electrode, a collector electrode and an emitter electrode.
When the switching characteristics of the power devices are measured under a wafer state through the use of the probe apparatus 110, the mounting table 112 holding the semiconductor wafer W is moved to bring the semiconductor wafer W on the mounting table 112 into electrical contact with the probe pairs 113A. If the power devices are turned on by the probe pairs 113A existing at the side of a gate G, a current corresponding to the voltage applied to the gate electrodes of the power devices flows from the cable 114 of the drain electrode (the collector electrode) to the source electrode (the emitter electrode).
If the cable 114 interconnecting the drain electrode (the collector electrode) of the mounting table 112 and the tester 115 become long, the inductance of the cable 114 grows larger and shows an increase of, e.g., 100 nH per 10 cm of the cable. For that reason, if the current change (di/dt) is measured on a microsecond unit through the use of the probe apparatus 110, the current change is small and is deviated from the ideal value as indicated by a broken line (an actual value) in FIG. 13. In the conventional probe apparatus 110, it is therefore difficult to accurately measure the current change (di/dt) inherent in the diode. In some cases, the diode is damaged. When turning off the power devices, an abnormal surge voltage is applied between the drain electrode (the collector electrode) and the source electrode (the emitter electrode). This may sometimes lead to damage of the power devices.